1. Field of the Invention
The present invention relates to a semiconductor device comprising an SRAM memory cell having two CMOS inverter circuits and serving as a memory device and a method of fabricating the same.
2. Description of the Related Art
A semiconductor device called “full CMOS static random access memory (SRAM)” comprises a plurality of memory cells each of which is composed of six MOS transistors. Arrangement patterns of point symmetry type, linear symmetry type and the like have been suggested for each memory cell. Occurrence of failure called “soft error” is recently at issue in SRAM's. In the soft error, externally incident neutron beam or αbeam changes the stored contents. The soft error has been found to be reduced by inserting a resistor or capacitor to a node of the SRAM memory cell circuit.
For example, U.S. Pat. No. 6,529,401 discloses an SRAM to which the aforementioned countermeasure is applied. In the disclosed countermeasure, a resistor is added to a node. Generally, an operating speed of the semiconductor device tends to be reduced when a resistor with a high resistance value is added to a node. However, since a higher priority is given to a reduction in power consumption rather than an operating speed in SRAM's of lower power consumption type, the aforesaid countermeasure can be sufficient.
In the countermeasure disclosed in aforementioned U.S. Pat. No. 6,529,401, a film is made from a material having a higher resistance value than cobalt silicide (CoSi), for example, tungsten (W) and patterned in order that a resistor with a high resistance value may be added to a node. In this case, the tungsten film is provided as a resistor electrically connected to the node. Consequently, the resistor has a higher resistance than silicide formed by a salicide process and accordingly, the aforesaid is an outstanding arrangement as a countermeasure against soft error. However, the aforesaid conventional arrangement requires an additional fabrication process of forming a tungsten film, thus increasing the number of processes. Consequently, the conventional arrangement increases the costs, it is difficult to employ actually.